Thursday, June 28, 2007

Monitoring the Brain in 3-D

An ultrasound endoscope could be used during surgery.

By Katherine Bourzac

Window on the brain: This image of blood vessels in a dog’s brain was made with a new 3-D ultrasound endoscope. The dog was injected with a contrast agent to color the vessels. The arrow points to one of the major blood vessels in the brain. Credit: Stephen Smith, Duke University

Duke University researchers have developed an ultrasound endoscope that will give surgeons a 3-D view of the brain during and after an operation. If it proves safe and effective in animal and human tests, the 3-D probe could provide a cheaper, more effective alternative to the two-dimensional scans currently in use. The probe has been used to image dogs' brains and will need to undergo clinical trials before it can be used in the operating room.

Currently, neurosurgeons rely on preoperative CT and MRI scans to orient themselves within a patient's brain during surgery. Once the brain is cut, the tissue shifts, and the scans may no longer be an accurate map. But performing these scans during surgery is impractical. The advantage of the new ultrasound method is that it's done in real time, says K. Kirk Shung, a biomedical engineer at the University of Southern California who isn't involved in the endoscope's development. "CT scans and magnetic resonance imaging are not real time, and are much more expensive."

Two-dimensional ultrasound is already widely used during biopsies to guide surgeons to tumors and during the implantation of devices such as electrodes for deep brain stimulation, says Duke bioengineer Stephen Smith, who developed the ultrasound device. But it's difficult to correlate the flat two-dimensional images with the reality of a patient's 3-D brain. And, Smith says, the existing ultrasound probes require the doctor to drill a large hole--one to two centimeters wide--in a patient's skull. His 3-D ultrasound probe requires a much smaller hole in the skull: less than a centimeter in diameter. Some procedures, such as biopsies or draining cerebrospinal fluid to relieve pressure on the brain, could be done through the same hole using the endoscope.

The Duke researchers tested the 3-D endoscope in dogs to image the vessels that carry the cerebrospinal fluid. They used a needle inserted into the endoscope to drain some fluid and inject a drug. They also tested the device in combination with a contrast agent to see the blood vessels in these dogs' brains in color.

Another potential use for the probe would be to help surgeons distinguish and remove tumor tissue. Studies using two-dimensional probes have demonstrated that doctors can differentiate many kinds of tumors using ultrasound imaging, says Smith.

The Duke device is a smaller variation on a previous 3-D ultrasound probe designed by Smith's lab for imaging the heart during surgery. "The daunting task is in miniaturization," says Smith, who developed the first 3-D ultrascanner for imaging the heart from outside the body in the late 1980s. "You have to run 100 to 500 wires down a thin tube." Ultrasound beams flow like water from a nozzle out of an array of about 500 transmitters and are picked up by about 250 receivers.

"I'm sure it will replace two-dimensional [brain] ultrasound if the image quality is better" than in the current system, says Shung. The resolution of the test device, he says, is not very high, but it could be tremendously improved by increasing the density of the probe's sensing array.

Meanwhile, Smith is working on making the 3-D ultrasound probes even smaller--small enough to fit inside a catheter that could be snaked into the brain through a blood vessel. This would eliminate the need to drill a hole in a patient's skull.